Size


One of the curious aspects of the diversity of life on our planet is that all large organisms are made of many, many cells, rather than being made of one great big cell. This suggests a couple of interesting questions, since a single cell can perform pretty much all of the necessary functions of life.

One of those questions is pretty basic. Why are there large organisms? What advantage does size have?

Cholera bacteria [colorized]

Before you provide the obvious answer, first consider the following: without question, the most successful life forms on Earth are the bacteria.

They outperform all other life forms in sheer numbers of organisms, probably numbers of species, and probably total biomass.If we think we are more successful than bacteria, we're fooling ourselves. Here's another thing: among animals, by far the most successful group are the insects, and despite some scary science fiction and horror movies, they just don't get very big (for a number of good biological reasons).

Elephant

So why are there big organisms? The answer to this question is one you've already encountered in this course. It has to do with the imperative that drives evolution: evolution is about diversity.

The true secret to survival is to be able to make a living in ways that other organisms aren't utilizing. If the world is packed full of cell-sized organisms, then there are ecological niches just waiting for organisms that are bigger, whole new ways to achieve the necessities of survival, and no competition — for a while, at least. Some of the neighbors always catch on, so no new innovation remains unique for very long.

That explains at least one reason why large organisms appeared. This leaves us with another puzzle. All of the large organisms on Earth are multicellular. There simply are no six foot tall unicellular organisms. Why? Since evolution is about diversity, why wouldn't this alternative be one that would work for some organisms?

There are at least two powerful answers to this question. One of them has to do with the problem of specialization. Probably for much the same reason that eukaryotic cells are compartmentalized, large organisms generally have a high degree of internal specialization. This is a lot easier to accomplish in a body formed of trillions of individual cells, rather than of one big hunk of protoplasm.

The other reason is one of necessity. It has to do with many of those basic functions of life we discussed in the introduction to this unit — things like gas exchange, getting food from the environment, getting rid of waste products. All of these processes are surface dependent. A cell acquires its food by transporting it across its plasma membrane, which covers its surface. Same with the exchange of oxygen and carbon dioxide — it all happens across that membrane. So the amount of food a cell can get is directly dependent upon how much surface area it has.

However, the entire cell has to be provided with food and oxygen, and the entire cell is producing waste materials and carbon dioxide. That means that the amount of food or oxygen needed is dependent upon the volume of the cell — how much space it occupies.

The dilemma is that the ability to perform these functions is surface dependent, but the need is volume dependent. So the amount of surface a cell has compared to how much volume it has is a vital aspect of its survival ability. If there is too little surface area for the cell's volume, it will starve and/or poison itself with its own waste materials.

So what does size have to do with this? Cells come in all shapes, but no matter the shape of an object, its surface area is calculated as a squared function, while its volume is calculated as a cubed function. This means that, as an object gets bigger, and both its surface area and its volume increase, the volume increases faster than the surface area does. Our six foot tall cell would be in desperate straits indeed. It wouldn't have nearly enough surface area to keep itself fed or to dump its waste products. One way to solve this problem is to make our six foot tall organism composed of a few trillion separate cells, each of which has the surface area to volume relationship of a tiny object, rather than a huge one.

Incidentally, there are other ways to approach these solutions, and though they don't work so well with the whole organism, other aspects of bodies use them extensively. This surface area to volume issue is important for more than cellular functions. The entire organism has problems to solve due to the same difficulty. For instance, consider your body. You have 60-100 trillion cells, all of which have to be supplied with nutrients and oxygen, and all of which are cranking out waste materials and carbon dioxide. Your whole body has to handle these needs. Unfortunately, your body's surface is gas and water proof. How are you going to supply all those cells with what they need? The answer to this dilemma is that you have specialized organs and organ systems which carry on all of these processes for your whole body. For example, your lungs manage gas exchange with the environment for all of the cells in your body. Of course, the lungs have to manage to have a huge amount of surface in them in order to get all of that gas exchange done. They achieve this by being divided into millions of tiny, thin walled sacs called alveoli, each of which is right next to capillaries (tiny blood vessels) with which to exchange oxygen and carbon dioxide. The result is a vast amount of surface area packed into a relatively small space. This is another way to solve this problem of not having enough surface area. Instead of having a simple surface, you make your surface very complicated. Later in this unit, you'll take a look at mitochondria and chloroplasts, both of which utilize this technique to pack huge amounts of membrane surface into a small space.

Oh, one last note. Remember those insects? Why don't they get big? At least part of the answer is that, in their evolutionary past, they turned in some directions that ended up limiting the size they could efficiently attain. beetle

One of those evolutionary "decisions" was the development of an exoskeleton (outside skeleton) as a support system. Vertebrates also developed a support system, but theirs is an endoskeleton (inside skeleton). An exoskeleton supports the animal from the outside; the soft tissues of the organism are attached by muscles to the inside of the exoskeleton. This works fine as long as the organism is fairly small. But as the size of the organism increases, the surface area to volume problem pops up. The amount of body which needs support is determined by the volume, but the size of the support system is determined by the surface area. The exoskeleton must increase in thickness (and clumsiness and weight) significantly faster than the soft body inside it does, just to be able to provide the same amount of support that a thin exoskeleton provided for a smaller insect. Organisms with endoskeletons don't really have this problem, because they are supporting their bodies from the inside, rather than from the outside.

Another way insect size has been limited is by the nature of the insect respiratory system. Remember the lungs we discussed above? Insects don't have lungs. The insect respiratory system involves many, many tiny tubes that run from the surface of the insect into the interior, providing gas exchange channels for the tissues inside the insect. Again, this works fine if the body is small, but if the insect's body gets too large, there simply isn't enough surface (and some of the interior is just too far away) for these little tubes to provide enough oxygen for the entire body of the inset.

So we were saved by a couple of the accidents of evolution. There will be no human eating flies outside the movie theatre. Well, at least no fly that can eat a human all by itself ;^)


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Updated 25 September 2004